Lori Cerasoli is the troubled star's "sober companion" and is on retainer to shadow Lohan's every move, from film sets to parties to shopping sprees - trying to make sure the rehabbed actress stays clean.

Cerasoli travels to jobs with Lohan, including the LA set of the film "Labor Pains" and the New York set of "Ugly Betty," in which Lindsay has a recurring role.

But she also reportedly hits clubs with the actress and bunks in Lohan's guesthouse or Lohan gal pal Samantha Ronson's spare room overnight.

"She's always there for Lindsay," a source close to Lohan said of Cerasoli. "She can talk to her if she ever feels like she needs a drink."

And Cerasoli came highly recommended. Lohan did a two-month rehab stint at Le Cirque Lodge last summer after two DWI arrests. People there hooked her up with Cerasoli last October.

Previously, Cerasoli had been Mary-Kate Olsen's $1,000-a-day eating coach after the rail-thin star underwent treatment at Cirque.

She monitored Olsen's eating habits, kept her away from drugs and alcohol, and even accompanied her on daily errands, such as apartment hunting in SoHo, according to Star magazine.

Cerasoli is a recovering alcoholic herself, says another star client, Drew Barrymore. In her 1991 tell-all, "Little Girl Lost," Barrymore described one group-therapy session in which Cerasoli turned to her after she had broken her sobriety and sternly said, "Answer this one simple question, 'Have you hit your bottom?' "

Barrymore was livid. But she answered "yes."

She also helped Barrymore after an attempted suicide in 1989.

Other celebs who have employed sober companions include Owen Wilson, Britney Spears, Mathew Perry, Robert Downey Jr., "The Insider" anchor Pat O'Brien and Robin Williams, according to reports.

"I give her most of the credit for the positive influence around my daughter," Michael Lohan said of Cerasoli.

(Source: NY Post)